1. Field of the Invention
The present invention relates to a data recording and playback device which records and reads out data from an optical recording medium, and a playback method of reading out data from an optical recording medium.
2. Description of the Related Art
Optical recording media which can accumulate high density data and which are capable of data processing at high speeds are attracting attention for audio and imaging uses, as well as for computer memory use. CDs, that are exclusively used for readout, are rapidly spreading for audio and computer uses. Moreover, 5.25 inch and 3.5 inch diameter (and the like) optical disks, on which it is possible to write data once (the "write once type"), and rewriteable types of magneto-optical disks, are standardized by ISO standards and are expected to be widely used also. Moreover, as for rewriteable optical disks, the phase change type has also started to appear on the market. Among such recording and playback devices, there is a strong desire to increase data storage capacity and to record and play back data at high density. Various investigations have been carried out for these purposes.
Playback light illuminates an optical recording medium from a playback device. When the intensity of the playback light increases (as shown in FIG. 5), the carrier to noise (C/N) ratio of the playback signal increases to some degree, corresponding to an increase of the amount of playback light which returns to the photodetector. However, when the playback light intensity is increased beyond a certain point corresponding to a C/N ratio maximum, due to a temperature rise of the medium, the C/N ratio begins to fall, due to a decrease of the Kerr rotation angle. Any further increase in the playback light intensity leads to the recorded data being destroyed (such as at the point identified by the data destruction arrow depicted in FIG. 5). In other words, the optimum intensity of playback light is present when the highest C/N ratio is obtained. Thus, in order to satisfy the requirement that information be recorded at high density and to exploit the maximum recording capacity of the medium, it is desirable to set the playback light intensity optimally to the best possible limits while addressing the increase of the C/N ratio of the playback signal.
Methods were attempted for making possible the playback of data which was recorded at high density by making the wavelength of the light source of the optical head short and using a small light spot for playback. But, the wavelength of the semiconductor laser used as a light source is limited. Moreover, at short wavelengths of the laser, there are problems of insufficient form or output of the laser light. Consequently, a medium was developed for super resolution readout use which can play back data recorded at high density, even if the present state of the wavelength of the light source and the size of the light spot used for playback stayed the same. In such a method, a portion of the signal of the medium which has entered the light spot is masked so that it is not detected as a playback signal by using a temperature distribution of the medium in the light spot arising from the combination of the temperature rise of the medium due to the playback light and from the rotational movement of the medium. As a result, the effective open aperture region which can read out a signal becomes smaller than the light spot, and playback of higher density data becomes possible.
Front Aperture Detection (FAD), which is an example of such a super resolution readout method, will be briefly described using FIGS. 6(a) and 6(b). FIG. 6(a) is a planar view of an optical recording medium used with a FAD system. FIG. 6(b) is a cross section at line A--A of FIG. 6(a). The medium 31, in accordance with a FAD system and method, is provided with three layers: a recording layer 32 made of, for example, TbFeCo; a disconnection layer 33 made of, for example, TbFe, and a playback layer 34 made of, for example, GdFeCo. Signal playback is performed from the playback layer side. The direction of magnetization of each layer in the initial state is as shown in FIG. 6(b). The direction of magnetization of the disconnection layer 33 and the playback layer 34 follow the direction of the recording layer 32 on which data (record marks 38) has been recorded. This is because exchange coupling forces act between mutually contacting layers.
An external magnetic field Hr is applied during playback of data. When a playback light spot 35 (as shown in FIG. 6(a)) moves relatively with respect to the medium 31, the region in the forward direction of the medium 31 introduced into the light spot 35 becomes a low temperature region 36, and the region in the rearward direction becomes a high temperature region 37, so that a temperature difference arises. Then, when the temperature of the high temperature region 37 reaches a temperature (Curie point) which erases the magnetization of the disconnection layer 33, the coupling of the magnetization of the playback layer 34 and the recording layer 32 via the disconnection layer 33 is cut off. In this manner, the magnetization of the playback layer 34 is reversed in the direction of the external magnetic field Hr (the direction of magnetization at the location B in FIG. 6(b) is reversed). That is, in the high temperature region 37, the magnetization of the playback layer 34 shows a constant state, regardless of whether there is a mark 38, and the high temperature region 37 becomes a mask which does not contribute to signal playback. On the other hand, only the low temperature region 36 which retains the recorded state provides an effective aperture for signal detection in the light spot 35. Accordingly, only the record mark 38a within the low temperature region 36 is read out.
In a medium for super resolution readout use, other than a medium for the FAD system, a Rear Aperture Detection (RAD) system and a Center Aperture Detection (CAD) system, having marks which remain in the open aperture of only the high temperature regions, are also known in the art.
The medium of the RAD system has a recording layer and a playback layer formed on it. By use of an initializing magnetic field, the direction of magnetization of the playback layer is made uniform in a constant direction. When illuminated by a light spot, the direction of magnetization of the playback layer reverses to the direction of magnetization of the playback layer in the high temperature region within the light spot by the exchange coupling force, and functions as an effective aperture for signal detection. Moreover, in the low temperature region, the direction of magnetization of the playback layer has the initial state and functions as a mask. Thus, super resolution readout is achieved in the RAD system.
A medium of the CAD system is provided with a recording layer and a playback layer which exhibits magnetization within the plane at a low temperature (horizontal direction of magnetization) and perpendicular magnetization at a high temperature. When the light spot illuminates the medium of a CAD system, the direction of magnetization of the playback layer, in the high temperature region close to the center within the spot, becomes the same direction as in the recording layer and functions as an effective aperture for signal detection. Moreover, in regions outside the high temperature region, the direction of magnetization of the playback layer is the unchanged magnetization in the plane. Because the magnetization in the plane does not confer a Kerr effect with respect to the perpendicular incident light, a mask effect is achieved. Thus, super resolution readout is achieved in the CAD system.
Accordingly, principles of the mask formation are represented by a method using the change of magnitude of the magnetic coupling force or the coercive force, or a method using the change of transmissivity due to phase change, and the like.
However, in the above-described methods for super resolution readout, the intensity of playback light which illuminates the optical recording medium from a playback device remains constant and has no relationship to the temperature of the medium and its surroundings. In the above-described mediums for super resolution readout use, when the temperature of the medium itself changes, even though illuminated with the same intensity of playback light, the temperature distribution of the medium within the light spot changes, and the above-described form of the effective aperture changes. Accordingly, when the temperature of the medium itself changed, the super resolution effect obtained may be inferior and readout errors may occur.
Moreover, in a normal medium (not a medium for use in super resolution readout), when the temperature of the medium itself changes, the C/N ratio of the playback signal changes, due to the aforementioned change in temperature distribution. Consequently, even though the playback light intensity was optimally set so as to obtain the highest possible C/N ratio, the best use could not be made of the capacity of the medium at constant playback light intensity because there was a fall in the C/N ratio when the temperature of the medium itself changed.
Furthermore, when the temperature of the optical recording medium was made normally constant on the playback device side, the established environmental temperature of the playback device was limited, countermeasures became necessary for the heat generated within the device, and the device became bulky.